Gamma ray imaging systems are employed in a variety of fields including nuclear medicine, homeland security, decommissioning of nuclear power plants, and astronomy. Conventional gamma ray imaging systems incorporate pixelated full-energy detectors for measuring the position and energy of gamma photons incident thereupon.
One commonly used system is a Compton camera, which consists of two pixelated detectors, a pixelated Compton scattering layer and a pixelated full-energy detector typically arranged at a distance from each other and parallel to each other. The Compton scattering layer serves to produce scattered gamma photons through Compton scattering of photons from gamma sources. Compton scattering is an inelastic scattering process in which a photon interacts with an electron, to produce a scattered photon. The electron absorbs a portion of the energy of the original photon such that the scattered photon is less energetic than the original photon. This energy difference is denoted the Compton energy shift. In addition, the propagation direction of the scattered photon differs from the propagation direction of the original photon by the Compton scattering angle. Energy and momentum conservation defines a correspondence between the Compton energy shift and the Compton scattering angle. The Compton scattering layer measures the position of a Compton scattering event as well as the associated Compton energy shift. The full-energy detector serves to measure the position and full energy of a scattered photon produced from Compton scattering in the Compton scattering layer. The Compton camera measures the position and Compton energy shift of a Compton scattering event in coincidence with the position and full energy of the scattered photon produced in the Compton scattering event. The locations of the gamma sources producing the original photons, as well as the energies of the gamma photons emitted by the gamma sources, are calculated from a number of coincidence measurements.
Since the Compton detector collects event data in coincidence, it has low background. However, pixelated full-energy detectors are expensive and suffer from low detection efficiency, unless manufactured with very large pixels, which further increases cost.
Another commonly used gamma ray imaging system is a coded aperture camera consisting of a coded aperture and a pixelated full-energy detector. The coded aperture is a substrate with several apertures arranged in a position-dependent pattern. Gamma photons emitted by a gamma source must pass through the coded aperture to reach the pixelated full-energy detector. The coded aperture leaves a position-dependent imprint on the image formed on the pixelated full-energy detector. A true image of the gamma source is reconstructed from the measured image and the known configuration of the coded aperture. The pixelated full-energy detector directly provides the energy spectrum of gamma photons emitted by the gamma source. Since the coded aperture camera does not use coincidence measurements, it may operate at higher detection efficiency than a Compton camera. However, the images tend to have more noise as the system does not use coincidence detection methods to filter out false events.